1. Field of the Invention
The present invention generally relates to a memory module and, more particularly, to the terminal structure of the memory module that can be removably inserted in a dedicated socket.
2. Description of the Prior Art
A memory module of a type adapted to be removably inserted in a dedicated socket mounted on a computer mother board has long been well known. Examples of the memory module of the type referred to above includes a DRAM module that is removably inserted in a SIMM socket or a DIMM socket.
FIG. 14 illustrates a schematic perspective view showing a memory module 100 and a printed circuit board 200 having a female socket 201 for the memory module 100 fixedly mounted thereon. The memory module 100 comprises a generally rectangular substrate 101 having a plurality of, for example, three memory chips 102, 103 and 104 mounted on at least one surface thereof in side-by-side fashion in a direction longitudinally of the substrate 101. One of the opposite side portions of the substrate 101, or a lower edge portion of the substrate 101 as viewed in FIG. 14, serves as a male connector 106. This male connector 106 includes an array of equally spaced terminal strips 105a positioned on one of the opposite surfaces of the substrate 101 and an array of equally spaced terminal strips 105b (See FIGS. 15 and 16) positioned on the other of the opposite surfaces of the substrate 101, each of said arrays of the terminal strips 105a and 105b extending in a direction longitudinally of the substrate 101. Each of the terminal strips 105a and 105b is generally in the form of a generally rectangular flat and thin conductor made of gold.
In the case of the 168-pins DIMM (Dual In-line Memory Module), 84 terminal strips, 1 mm in width, are arrayed at the pitch of 0.27 mm on each of the opposite surfaces of the substrate 101 adjacent the lower side thereof. On the other hand, in the case of the 144-pins DIMM, 72 terminal strips, 0.5 mm in width, are arrayed at the pitch of 0.3 mm on each of the opposite surfaces of the substrate 101 adjacent the lower side thereof.
Referring to FIG. 15 which illustrates a cross-section of the memory module 100 and the printed circuit board 200 taken along the line XV-XV' in FIG. 14, the female socket 201 mounted on the printed circuit board 200 includes an array of terminal tongues 202, equal in number to and engageable with the terminal strips 105a, and another array of similar terminal tongues 203 equal in number to and engageable with the terminal strips 105b are formed in respective interior walls of the female socket 201 that are opposed to each other.
In this known terminal structure, electrically insulating foreign matter 110 such as frush or fragment may often be found resting on or adhering to one or some of the terminal tongues 202 and/or 203. If while the foreign matter 110 is found resting on or adhering to one of the terminal tongues 202 as shown in FIG. 15 an attempt is made to insert the memory module 100 into the female socket 201, there is a high possibility that one of the terminal strips 105 which is to be electrically connected with such one of the terminal tongues 202 will fail to establish a circuit with the mating terminal tongue 202 with the foreign matter 110 jammed between the terminal strip 105 and the terminal tongue 202 as shown in FIG. 16.
The Japanese Laid-open Patent Publication No. 7-231153, published Aug. 29, 1995, discloses a card edge substrate having an edge portion adapted to be removably inserted in a dedicated female socket. At least one surface of the card edge substrate is formed with a plurality of juxtaposed card edge terminals spaced an equal distance from each other. Each of the card edge terminals has a tip terminating at a location spaced a slight distance inwardly from the edge of the card edge substrate. An elongated solder resist layer in the form of an elongated layer of synthetic resin is deposited on the edge portion of the card edge substrate so as to cover the respective tips of the card edge terminals. This solder resist layer is adapted to be trapped by flexible terminal tongues in the female socket to avoid an fall-off or accidental separation of the card edge substrate from the female socket.
In this known terminal structure, as the card edge substrate is inserted into the female socket with the edge thereof received within the female socket, the elongated solder resist layer relatively slides over the terminal tongues while the latter are urged against their own resiliency to allow the solder resist layer to pass and is subsequently caught by the terminal tongues then restoring to the original shape by the effect of their own resiliency.
The publication No. 7-231153 describes that the elongated solder resist layer is made of the synthetic resin which, when cured, provides a minimized resistance to the terminal tongues that pass in sliding contact therewith and is also so shaped as to minimize the possibility of an fall-off or accidental separation of the card edge substrate from the female socket.
However, it has been found that the elongated solder resist layer disclosed in the publication No. 7-231153 is ineffective to remove the foreign matter resting on one or some of the terminal tongues in the female socket and, hence, ineffective to eliminate the possible connection failure.
The Japanese Laid-open Patent Publication No. 9-7709, published Jan. 10, 1997, discloses the use of stepwise cutouts in the edge of the card or substrate so that the force required to push the card into the female socket or socket can be minimized. Considering, however, that the card or substrate disclosed in the publication No. 9-7709 MAKES use of flat and thin conductors similar to the terminal strips 105 and 105b shown in FIGS. 14 to 16, the terminal structure is ineffective to eliminate the foreign matter resting or adhering to one or some of the socket tongues.